Treating aluminium with chlorine

Abstract

When molten aluminium is treated with chlorine to remove alkali and alkaline earth metals and hydrogen, there is formed a brittle oxide crust which does not protect the metal from oxidation. This problem is solved by admixing with the chlorine a minor proportion of a gaseous compound of fluorine, preferably sulphur hexafluoride, which can form alkali or alkaline earth metal fluorides in the presence of molten aluminium. The proportion of sulphur hexafluoride to chlorine gas is preferably from 0.01 to 1.0. Ingots cast using the treated metal are free of oxide patches on their surface.

We claim: 
     
       1. In a method of treating a molten aluminium alloy in which magnesium is present in a concentration of at least 1% and is the principal alloying element by continuous in-line fluxing with chlorine gas to remove hydrogen and/or alkaline metal impurities from the molten alloy wherein the said fluxing of the molten alloy with chlorine gas also causes the formation of magnesium chloride in the molten alloy, the improvement wherein the said fluxing is carried out using chlorine gas pre-mixed prior to said fluxing with a gaseous compound of fluorine other than a halogenated hydrocarbon whereby surface oxidation of said magnesium chloride formed during said fluxing is substantially eliminated during subsequent transfer and/or casting of said alloy, the proportion by volume of the gaseous fluoride compound to chlorine gas used being from 0.01 to 1.0. 
     
     
       2. A method as claimed in claim 1, wherein the gaseous compound of fluorine is sulphur hexafluoride. 
     
     
       3. A method as claimed in claim 1, wherein the chlorine and the compound of fluorine are diluted by an inert carrier gas. 
     
     
       4. A method as claimed in claim 1, including the step of casting the treated metal in a casting mould to form an ingot, wherein the treatment is carried out in such a way that oxide patches are not formed within the casting mould on the molten metal surface, nor on the surface of the ingot during the casting operation. 
     
     
       5. A method as claimed in claim 1, wherein the gaseous compound of fluorine is silicon tetrafluoride. 
     
     
       6. A method as claimed in claim 1, including the step of casting the treated molten alloy in a mould within which a surface of the molten alloy is exposed such that oxidation can occur at said surface.